The invention relates to fused silica. More particularly, the invention relates to a method of loading fused silica with hydrogen.
Molecular hydrogen is desirable in fused silica lenses used in photolithography because it mitigates color center formation caused by excimer laser radiation. Fused silica made by the soot-to-glass process does not contain molecular hydrogen, and therefore requires that hydrogen be loaded into the fused silica after sintering/consolidation.
Hydrogen easily diffuses into fused silica. However, at low temperatures, loading times are inconveniently long. At higher temperatures, hydrogen reacts with the silica lattice to form silicon hydride (SiH*) at unacceptably high concentrations. The presence of SiH* is undesirable, as it decreases initial transmission, transient absorption, increases fluence, and may contribute to laser induced wavefront distortion. The degree of reaction of silicon with hydrogen depends upon loading time and temperature, as well as the composition of the fused silica.